A wind power plant can be a single grid-connected unit but usually consists of a number of wind turbines forming a wind power farm. Each wind turbine is equipped with an electric generator located in a hub. The generator can be synchronous or of the induction type. Induction generators are more common today because they are cheaper and more robust. The synchronous generator can produce reactive power which is an advantage over the induction machine. The size of the wind turbine is today typically 100-3000 kW with many commercial turbines around 500 kw. The trend is for higher power and voltage of the generator. The voltage levels of today are from 400 V up to a few kV. In most wind farms, it is necessary to equip each wind turbine with a transformer that steps up the voltage to a local distribution voltage that may be typically 10-30 kV. Thus this transformer and the generator constitute integrated parts of a plant. Individual units are interconnected in tree branch or ring networks with high-voltage cables. The distribution network may be connected to a transmission network by a single or a couple of power transformers. The transformers entail an extra cost and also have the drawback that the total efficiency of the system is reduced. They are also a fire hazard since they contain transformer oil which can leak out in the event of failure or vandalism.
If, therefore, it were possible to manufacture electric generators for considerably higher voltages, at least the distribution transformer could be eliminated. It is possible with today's generator technology to make a 10 kV generator and thus eliminate the distribution transformer, but the cost would be far higher than a more typical 660 V machine. Furthermore today's stator winding insulation technology is sensitive to temperature variations, humidity and salt that a wind turbine generator may be exposed to. This makes it unrealistic with today's technology to dispose of the distribution transformers.
A high-voltage generator has a magnetic circuit that comprise a laminated core, e.g. of sheet steel with a welded construction. To provide ventilation and cooling the core is often divided into stacks with radial and/or axial ventilation ducts. The winding of the magnetic circuit is disposed in slots in the core, the slots generally having a cross section in the shape of a rectangle or trapezium.
In multi-phase high-voltage electric generators the windings are made as either single or double layer windings. With single layer windings there is only one coil side per slot, whereas with double layer windings there are two coil sides per slot. By “coil side” is meant one or more conductors combined vertically or horizontally and provided with a common coil insulation, i.e. an insulation designed to withstand the rated voltage of the generator to earth.
Double-layer windings are generally made as diamond windings whereas single layer windings in the present context can be made as diamond or flat windings only one (possibly two) coil width exists in diamond windings whereas flat windings are made as concentric windings, i.e. with a widely varying coil width. By “coil width” is meant the distance in arc dimension between two coil sides pertaining to the same coil.
Normally all large machines are made with doublelayer windings and coils of the same size. Each coil is placed with one side in one layer and the other side in the other layer. This means that all coils cross each other in the coil end. If there are more than two layers these crossings complicate the winding work and the coil end is less satisfactory.
It is considered that coils for rotating generators can be manufactured with good results within a voltage range of 3-20 kV.
In theory, it is known how to obtain larger voltage levels. Such generators are described, for instance, in US-A-4429244, US-A-4164672 and US-A-3743867. However, the machine designs according to the above publications do not permit optimal utilization of the electromagnetic material in the stator.
There are also wind turbines that operate at variable turbine speed. This operation mode is advantageous because the aerodynamic efficiency can be maximized. Variable speed systems employ two generators with different numbers of poles or generators with windings that can be connected for two-speed operation. Variable speed can also be obtained by means of a frequency converter. A variable speed system is simplified when a synchronous generator is used because a simple diode rectifier can be used between generator and DC-link. The two most common inverter types are line commutated and force-commutated. These two types of inverters produce different types of harmonics and hence require different line filters. The line-commutated inverter is equipped with thyristors which produces harmonic current that are turned into voltage harmonics on the grid. To eliminate these harmonics a large grid filter must be used. Another drawback is that the line-commutated inverter consumes reactive power. A force-commutated inverter can create its own three-phase voltage system and if the inverter is connected to the grid it can freely choose which power factor to use and in which direction the power should be directed. By the use of Pulse Width Modulation, PWM, the low frequency harmonics are eliminated and the first harmonics have a frequency around the switching frequency of the inverter. The most interesting valve for a PWM inverter is the insulated Gate Bipolar Transistor, IGBT. With the latest IGBT-valves, a switching frequency of from 5 to 10 kHz would be used. Today's IGBT valves are limited in voltage and power so that a single six-pulse inverter can handle about 1 MVA at 1-2 kV.